High quality colloidal nanocrystals are nanometer sized fragments formed in solution with well-controlled size, shape, surface structures, and excellent chemical processibility. As used herein, chemical processibility means that nanocrystals can be treated as solution species. Colloidal nanocrystals are of great interest for industrial applications and academic studies because of their unique size dependent properties and flexible processing chemistry.
Colloidal nanocrystals, particularly of semiconductor materials, continue to exhibit tremendous promise for developing advanced materials, and have attracted great interest for their utility in fundamental research. These nanocrystal-based emitters can be used for many purposes, such as light-emitting diodes, lasers, biomedical tags, photoelectric devices, solar cells, catalysts, and the like. However, the lack of adequate synthetic methods for preparing high quality nanocrystals has hampered progress in this area, and delayed the timely development of advanced applications for these unique materials. Present synthetic schemes for semiconductor nanocrystals, including various organometallic approaches and their inorganic alternatives, are sometimes irreproducible and often provide crystals that are low in quality, possess high polydispersities, and may be plagued by impurities.
Many current preparative methods require the use of toxic, pyrophoric, and unstable reagents. For example, the synthesis of CdSe nanocrystals using dimethyl cadmium (Cd(CH3)2) as the cadmium precursor is now well developed (Murray et al., J. Am. Chem. Soc. 1993, 115, 8706–8715; Barbera-Guillem, et al., U.S. Pat. No. 6,179,912; Peng et al., Nature 2000, 404, 69–61; Peng et al., J. Am. Chem. Soc. 1998, 120, 5343–5344). However, dimethyl cadmium is extremely toxic, pyrophoric, expensive, and unstable at room temperature. At the typical injection temperatures (340–360° C.) required for nanocrystal synthesis using Cd(CH3)2 as the precursor, Cd(CH3)2 is explosive by releasing large amounts of gas. For these reasons, the Cd(CH3)2 related synthesis methods require very restrictive equipment and conditions and, thus, are not ideal for large-scale synthesis.
Another limitation in current preparative methods for nanocrystals is their general inability to provide monodisperse samples. Currently, CdSe is the only compound for which nanocrystals having a relatively monodisperse size distribution can be directly synthesized (Peng, et al., J. Am. Chem. Soc. 1998, 120, 10, 5343–5344). Peng, et al. reported that the size distribution of CdSe nanocrystals can approach monodispersity (polydispersity index, PDI≈1), by controlling the monomer concentration in the initial reaction solution, and that CdSe nanocrystal size could be controlled by adjusting the time for crystal growth. There, thus, remains a need to develop a more generally applicable method for synthesizing high-quality semiconductor nanocrystals, whereby the size and size distribution of the nanocrystals can be well controlled during the growth stage (“focusing” of the size distribution).
Recently, Peng reported that the formation of high quality CdSe nanocrystals can be achieved by the use of stable, inexpensive, and safe inorganic cadmium salts, instead of dimethylcadmium (Peng, et al., J. Am. Chem. Soc., 2001, 123, 168; 2002, 124, 3343; Qu, et al., Nanoletters, 2001, 1, 333; J. Am. Chem. Soc., 2002, 124, 2049; U.S. patent application Ser. No. 09/971,780; U.S. Provisional Patent Application No. 60/275,008). However, the synthesis was performed in coordinating solvents, which resulted in limited success for the growth of high quality CdTe and CdS nanocrystals (Peng, et al., J. Am. Chem. Soc., 2001, 123, 168).
Current synthetic methods for high quality semiconductor nanocrystals, as discussed above, are exclusively performed in coordinating solvents, based upon the general belief that such solvents are necessary to adequately dissolve and allow complete reaction of their synthetic precursors. However, while long thought necessary, coordinating solvents suffer from several limitations as reaction media for synthesizing semiconductor nanocrystals. For example, the coordinating ability of representative solvents is often limited, making it very difficult to identify a good solvent system for a specific synthesis. Likely, this feature has limited the quality of available CdSe nanocrystals for many years. Further, coordinating solvents are often quite expensive, which may hinder large scale development efforts of an otherwise acceptable synthetic method. Many common coordinating solvents are toxic, and safety considerations may effectively preclude large scale syntheses. Thus, simply identifying a coordinating solvent with the necessary physical properties can be quite involved, thereby complicating the search for a suitable reaction system for growing high quality nanocrystals for most semiconductor materials.
Attempts to address these limitations have led to the general practice of using a mixture of several coordinating reagents as solvent. However, this practice also presents the non-trivial challenge of identifying an appropriate solvent system for crystal growth. Further, mixed solvent systems make it very difficult to identify the role of each component of the coordinating solvent in the growth of nanocrystals, which places further developments in this area on a highly empirical, rather than a more rational, basis. Moreover, such complicated reaction systems preclude so-called “green” chemical syntheses, because of the difficulty in recycling the raw materials and the toxicity of the most popular coordinating solvents, such as organophosphorus compounds.
Therefore, what is needed is an improved method to prepare semiconductor materials that affords high quality and pure nanocrystals. This method would also avoid the toxic solvents commonly used, and provide more green approaches to these nanocrystals using more recyclable solvents. Prefereably, the improved method would be amenable to syntheses in the air, rather than requiring an inert atmosphere, and it would use solvents that are liquid at room temperature to provide lower costs relative to current methods. If possible, the new method would also impart the ability to control the size of the nanocrystals produced, without sacrificing the desired narrow size distribution.
The present invention demonstrates that, despite the general belief that coordinating solvents are necessary for preparing semiconductor nanocrystals, these materials may in fact be prepared in non-coordinating solvents. Therefore, this invention exhibits the desired features described above by providing synthetic methods that produce high quality, small, and highly monodisperse semiconductor nanocrystals.